Beeline for Go v0.2.0

Honeycomb's Beeline for Go has gotten an overhaul! Our Beeline now comes with support for seamless tracing out of the box - the data model no longer simply supports the concept of "one current span," it now supports a doubly-linked tree of events (now dubbed "spans") representing a trace.

To see these changes in use, check out the new function calls StartSpan, AddFieldToTrace, and the wrapped RoundTripper! (See them all together in a single example.)

For more detail on the 0.2.0 release, see the repo's CHANGELOG.